Rubber dome pressure actuated switches are often used in field consoles and other telephone applications, most often over rigid circuit boards, especially where the use of hard plastic keys or hard plastic key caps are not practical, or where their incorporation into a push button type switch is not economically feasible. In addition, size and space restrictions often make the use of hard plastic caps unfeasible. It is often the case that sealants are used to impregnate the rubber dome, but the use of such materials and the process of impregnation are too costly, especially where the number of switches is extremely large, as in most telephone applications involving, for example, telephone consoles or terminals.
Heretofore, circuit board assemblies using rubber dome keys have been subject to mechanical and electrical failures in large numbers over an extended period of use, or where the use, i.e., actuation of the switch by an operator pressing on the dome, is of a high frequency of occurrence. Such failures are costly, especially in the field, both as to replacement of the defective switch and also as to repair of the switch or the circuit board itself. Thus, in prior art arrangements, any defects necessitating repair of the switch generally require replacement of the entire circuit board to which the switch is affixed.
Recent studies have shown that the circuit board containing the switch pads and upon which the switches are mounted and, more particularly, the switch pads themselves, are being contaminated during use by a foreign substance which chemical analysis has shown to be squalene, a salt that is expelled through the skin of the human body, most often as a component of sweat. Squalene is also found in some types of cosmetics and is thus quite prevalent. This substance or salt has an oily consistency and a very high viscosity which allows it to be readily absorbed by the silicone rubber of the domes. Over a period of time or of frequent operation, the rubber dome becomes saturated with the salt and, upon continued actuation of the switch, the salt becomes deposited on the circuit board, especially the switch pads, and almost invariably leads to failure of the switch pads or even of other components on the circuit board. One type of rubber dome switch that is susceptible to such contamination is shown in U.S. Pat. No. 4,818,829 of Nopper et al. In FIG. 2 of that patent, there is shown a structure wherein the dome, upon actuation, i.e., depression, bears against an active component of the circuitry on the circuit board. Over time, the squalene contamination will reach and probably contaminate this element.
In present usage, dome switches, or other types of pressure actuated switches, are made up of a plurality of parts which are generally bonded together for both stability and reliability. Such structures are shown in the aforementioned Nopper et al. patent as well as in U.S. Pat. No. 4,818,827 of Ipeinski, et al., which shows a bonded membrane type switch. As discussed heretofore, when contamination, deterioration, or other malfunction producing event occurs, the entire switch, as exemplified by the aforementioned patents, along with its companion circuit board, generally has to be replaced. It can be appreciated that, where, for example, fifty switches are mounted on the circuit board and only one switch causes a malfunction, such a replacement procedure can be extremely expensive. When, as in most of the prior art arrangements, a bonded type switch is used, there is no alternative to replacing the entire inseparable array of switches and other elements.